Method of manufacturing an interference coating

ABSTRACT

A method of manufacturing an interference coating on the surface of an aluminum alloy or aluminum alloys product comprising anodizing and electrochemical dyeing with use of alternating current consisting in that, the electrolyte used during electrochemical dyeing comprises copper (II) sulfate (IV) in an amount from 1 to 100 g/L, boric acid in the amount of 1 to 40 g/L and tartaric acid in the amount of 0.1 to 20 g/L.

TECHNICAL FIELD

The object of the present invention is a method of manufacturing an interference coating on the surface of an aluminum alloy or aluminum alloys product comprising anodizing and electrochemical dyeing with use of alternating current.

BACKGROUND ART

Due to the favorable strength to specific weight ratio and the ease of forming, aluminum alloys have gained popularity in many manufacturing industries. Currently, aluminum alloys are widely used in the automotive, construction and food packaging industries. Because of the conditions in which the products operate it is necessary to additionally protect the metal surface against adverse external factors. For some industries, the visual effect is a key factor in the customer's decision to purchase a given product. The growing awareness of the need for sustainable development poses new challenges for engineers working on the development of new coatings. Currently, the market requires meeting all three of the above-mentioned aspects, the coating must meet certain technical requirements, be environment friendly and offer unprecedented visual effects.

The protection of the aluminum surface can be achieved by anodizing process. The process itself is well known and described in the available literature.

The document U.S. Pat. No. 1,869,058 from 1931 describes anodizing process by using low voltage in sulfuric acid. Over the years many works appeared presenting different approaches to the development and improvement of the technique in order to meet the latest environmental and consumer requirements.

Dyeing process without preceding anodizing step is described in the document U.S. Pat. No. 4,115,212. The document discloses electrochemical dyeing process of aluminum alloy objects using an alternating current in an aqueous bath consisting of sulfuric acid and boric acid or only sulfamic acid and salts of metals. The process ends with additional protection of the surface by applying a colorless varnish. The layer, however, has no interference effect.

The document U.S. Pat. No. 4,066,816 describes anodizing process combined with electrochemical dyeing which results in an interference coating. The document describes two-step anodizing in sulfuric acid and in phosphoric acid, which necessitates the rinsing of the elements in order to avoid transfer of the bath between the stages. The second anodizing is to enlarge the pores in the oxide layer.

In the scientific publication Synthesis and properties of iridescent Zn-containing anodic aluminum oxide films (Thin Solid Films 586 (2015) 8-12) electrochemical deposition of zinc atoms from a solution of 80 g/L ZnSO₄·7H₂O and 20 g/L H₃BO₃ was preceded by anodizing in phosphoric acid for at least 10 minutes. In the work Interference Coloring of Dual-Anodized Films on Aluminum Containing Electrolytically Deposited Thin Metal Layers (Plating and Surface Finishing; 84, 5; 116-119; 1997) the interference effect was obtained by depositing tin and nickel atoms in the pores of aluminum oxide.

The document WO2019011778 describes a method of manufacturing an iridescent coating on rolled products. Anodized thin, porous aluminum oxide layer with a thickness of 15 to 25 nm is dyed with azo, anthraquinone or indigo dyes. This process is completed with a coating obtained by a sol-gel method. The layer obtained in this way is characterized by a slight iridescent effect.

DISCLOSURE OF THE INVENTION

A method of manufacturing an interference coating on the surface of an aluminum alloy or aluminum alloys product comprising anodizing and electrochemical dyeing with use of alternating current according to the present invention is characterized in that, the electrolyte used during electrochemical dyeing comprises copper (II) sulfate (VI) in an amount from 1 to 100 g/L, boric acid in the amount of 1 to 40 g/L and tartaric acid in the amount of 0.1 g to 20 g/L.

Dyeing is conducted under continuous stirring of electrolyte.

Time of dyeing is longer than 10 seconds.

Dyeing is conducted at a temperature 5-40° C.

Dyeing is conducted with use of alternating current having voltage in range 0.5-50 V.

Anodizing is conducted in the solution of sulfuric acid (VI) having concentration of 50-500 g/L with the addition of aluminum ions in the amount of 0-100 g/L.

Anodizing is conducted at a current density of 0.1-5 A/dm².

Anodizing is conducted for 10-3600 seconds.

The product is sealed after dyeing.

Sealing is conducted by hot or cold hydrothermal or vapor deposition methods.

Sealing is conducted by vapor deposition method using atomic layer deposition.

In order to ensure good and reproducible quality of coatings, the surface of the aluminum product should be degreased and etched before starting anodizing. This process can take place in baths based on organic solvents, acidic and alkaline water baths. After washing and etching the workpiece, rinse it with deionized water. After excess water has drained off, the aluminum product can be subjected to the process of manufacturing an interference coating. The above-described method of preparing the surface of the product can be replaced by another known method. Other alternative methods are available in the state of the art that can ensure adequate surface quality.

After anodizing, the product should be rinsed, preferably in deionized water, and then transferred to the dyeing bath.

The unquestionable advantage of the method of obtaining the interference coating according to the invention is appropriate selection of the electrolyte components. Use of copper (II) sulfate (VI) is much safer in comparison to the previously used electrolytes, e.g. cobalt sulfate (VI) or nickel sulfate (VI), which are suspected of and/or show carcinogenic properties. The composition of the electrolyte allows to obtain wide range of color variants in one bath, thus facilitating the technological process, also shortening the time of creating the coating, which is an important economic aspect and is beneficial for the natural environment as it reduces energy consumption.

The method according to the invention may in particular be useful for obtaining a new graphic effect on the surface of beverage cans or their lids.

The subject of the invention is disclosed at the drawings, where FIG. 1 illustrates a block diagram of the interference coatings manufacturing process, FIG. 2 —cross-section of the aluminum product showing the structure of the coating with light interference.

EXAMPLES Example 1

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 150 g/L with the addition of aluminum ions at the level of 1.1 g/L at a temperature of 18° C., under direct current conditions with a current density of 1.1 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 200 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 8 V in an electrolyte containing 15 g/L of copper (II) sulfate (VI), 20 g/L of boric acid and 1 g/L of tartaric acid. During the whole process, the electrolyte temperature is 18° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 120 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried.

The block diagram of the interference coating manufacturing process is shown in FIG. 1 , where the numbers represent

-   -   1—the process of washing and etching the aluminum product;     -   2, 4, 6, 9—rinsing process with deionized water;     -   3—anodizing of the aluminum product;     -   5—dyeing with alternating current (AC);     -   7, 10—drying the finished product;     -   8—sealing the oxide coating on the aluminum product.

The dashed line indicates the optional sealing path for the coating of the aluminum product.

Example 2

Manufacturing of an interference coating according to the guidelines described in example 1, followed by a hydrothermal heat seal. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 3

Manufacturing of an interference coating according to the guidelines described in example 1, followed by cold sealing. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 4

Manufacturing of an interference coating according to the guidelines described in example 1, followed by, and then performing the seal using vapor deposition method e.g. atomic layer deposition method. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 5

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 145 g/L with the addition of aluminum ions at the level of 1 g/L at a temperature of 11° C., under direct current conditions with a current density of 1.2 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 250 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 12 V in an electrolyte containing 13 g/L of copper (II) sulfate (VI), 22 g/L of boric acid and 1.6 g/L of tartaric acid. During the whole process, the electrolyte temperature is 22° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 160 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 6

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 150 g/L with the addition of aluminum ions at the level of 1.1 g/L at a temperature of 18° C., under direct current conditions with a current density of 1.1 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 200 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 8 V in an electrolyte containing 15 g/L of copper (II) sulfate (VI), 22 g/L of boric acid and 1.6 g/L of tartaric acid. During the whole process, the electrolyte temperature is 18° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 120 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 7

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 120 g/l with the addition of aluminum ions at the level of 1 g/L at a temperature of 18° C., under direct current conditions with a current density of 1.1 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 200 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 8 V in an electrolyte containing 15 g/L of copper (II) sulfate (VI), 20 g/L of boric acid and 1 g/L of tartaric acid. During the process, the electrolyte temperature is 18° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 120 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 8

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 150 g/L with the addition of aluminum ions at the level of 1 g/L at a temperature of 20° C., under direct current conditions with a current density of 1.1 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 200 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 8 V in an electrolyte containing 15 g/L of copper (II) sulfate (VI), 20 g/L of boric acid and 1 g/L of tartaric acid. During the whole process, the electrolyte temperature is 22° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 120 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 9

After degreasing and etching the surface, the aluminum product is anodized in an aqueous solution of sulfuric acid (VI) having concentration of 120 g/l with the addition of aluminum ions at the level of 1 g/L at a temperature of 21° C., under direct current conditions with a current density of 1.1 A/dm². An aluminum sheet with a surface equal to or greater than that of the aluminum product may be used as the cathode. The anodizing time was 250 seconds. After completing the anodizing process, the product should be rinsed in deionized water, and after draining off the excess, next step can be started. Electrochemical dyeing is performed using an alternating current of 8 V in an electrolyte containing 15 g/L of copper (II) sulfate (VI), 20 g/L of boric acid and 1 g/L of tartaric acid. During the whole process, the electrolyte temperature is 25° C. A stainless steel counter electrode has a working surface equal to or greater than that of the aluminum product. The electrolyte is constantly stirred and the dyeing time is 100 seconds. After the end of the process, the aluminum product is rinsed in deionized water and then dried. The block diagram of the interference coating manufacturing process is shown in FIG. 1 .

Example 10

The aluminum product manufactured by the method according to the invention has an interference coating. FIG. 2 shows a cross-section of such a product showing the structure of a coating in which light interference occurs, where the individual numbers represent:

-   -   11—area of precipitated copper;     -   12—pore in the structure of the oxide layer on aluminum;     -   13—an oxide layer on aluminum resulting from the anodizing         process;     -   14—the wall of the aluminum product;     -   A—ray of light reflected from the oxide layer;     -   B—a ray of light reflected from the surface of copper         precipitation;     -   C—a ray of light reflected from the bottom of the porous         structure;     -   d—the thickness of the oxide layer on the aluminum. 

1. A method of manufacturing an interference coating on the surface of an aluminum alloy or aluminum alloys product comprising anodizing and electrochemical dyeing with use of alternating current, characterized in that, the electrolyte used during electrochemical dyeing comprises copper (II) sulfate (VI) in an amount from 1 to 100 g/L, boric acid in the amount of 1 to 40 g/L and tartaric acid in the amount of 0.1 to 20 g/L.
 2. The method according to claim 1, characterized in that, dyeing is conducted under continuous stirring of electrolyte.
 3. The method according to claim 1, characterized in that, the time of dyeing is longer than 10 seconds.
 4. The method according to claim 1, characterized in that, dyeing is conducted at a temperature 5-40° C.
 5. The method according to claim 1, characterized in that, dyeing is conducted with use of alternating current having voltage in range 0.5-50 V.
 6. The method according to claim 1, characterized in that, anodizing is conducted in the solution of sulfuric acid (VI) having concentration of 50-500 g/L with the addition of aluminum ions in the amount of 0-100 g/L.
 7. The method according to claim 1, characterized in that, anodizing is conducted at a current density of 0.1-5 A/dm².
 8. The method according to claim 1, characterized in that, anodizing is conducted for 10-3600 seconds.
 9. The method according to claim 1, characterized in that, the product is sealed after dyeing.
 10. The method according to claim 9, characterized in that, sealing is conducted by hot or cold hydrothermal or vapor deposition methods.
 11. The method according to claim 10, characterized in that, sealing is conducted by vapor deposition method using atomic layer deposition. 